In a time-of-flight mass spectrometer (TOF-MS), the mass of an ion is generally calculated from the time of flight which is obtained by measuring a period of time required for the ion to fly at a fixed distance, on the basis of the fact that an ion accelerated by a fixed energy has a flight speed corresponding to the mass of the ion. Accordingly, elongating the flight distance is particularly effective to enhance the mass resolution. However, elongation of a flight distance on a straight line requires unavoidable enlargement of the device, which is not practical, so that a mass spectrometer called a multi-turn time-of-flight mass spectrometer has been developed in order to elongate a flight distance.
A multi-turn ion optical system for making ions turn in such a multi-turn time-of-flight mass spectrometer generally has a closed orbit and a unit structure having a time-focusing property (refer to Non-Patent Document 1, for example). To “time-focus” in the present invention means that the time of flight of the ions is not dependent on an initial position, initial angle, and initial energy of the beam of the ions in a first-order approximation. As a component of the multi-turn ion optical system, a sector-formed electric field which has a simple configuration and good versatility is often used. In a multi-turn time-of-flight mass spectrometer as described in Patent Document 1 for example, the flight distance is effectively elongated and the mass resolution of ions is enhanced by forming an approximately figure-eight “8” shaped loop orbit using a plurality of sector-formed electric fields and causing ions to fly along this loop orbit repeatedly multiple times.
In such a mass spectrometer, an ion source for generating ions and an ion detector for detecting ions may be placed on the loop orbit in some cases. However, in many cases, ions generated outside the loop orbit are injected to the loop orbit to fly for a predetermined number of turns, and the ions are deviated from the loop orbit to be introduced to an ion detector provided outside of the loop orbit to be detected. In the apparatus described in Patent Document 1, in order to inject ions to and eject ions from the loop orbit, an opening through which ions can pass is bored in a sector-formed electrode, and the sector-formed electrode is driven in a pulsed manner to inject ions linearly to the loop orbit. In the same manner, ions are ejected from the loop orbit.
In such a manner of injecting and ejecting ions, the variation of the energy of ions is not time-focused in a linear free flight space for injection and ejection, and therefore, when looking at the entire path that ions pass from the starting point of the ions (usually an ion source) to the detection point of the ions (usually an ion detector), the time-focusibility that a multi-turn ion optical system originally has is not assured. This contributes to a decrease in the accuracy of analysis.
This manner requires the connection of a power supply which can supply pulses to the sector-formed electrodes composing a multi-turn ion optical system which can be statically driven (i.e. a direct-current (DC) voltage is applied) in order to cause ions to fly along the loop orbit. This makes it difficult to ensure the stability of the DC voltage applied to the sector-formed electrodes from the power supply, which might exert a negative effect on the accuracy of analysis. In addition, the necessity of preparing such a power supply for supplying pulses and a stable DC voltage increases the cost.
Another method for injecting ions to and ejecting them from a multi-turn ion optical system is to add a sector-formed electric field for the ion injection and for the ion ejection respectively, as described in Non-Patent Document 2. However, in an injection/ejection ion optical system including the added sector-formed electric fields, the time focus at the original time-focusing point of the multi-turn ion optical system is not considered; only the time focus when ions pass each of the injection ion optical system and the ejection ion optical system is insufficiently achieved. Therefore, in order to ensure the time-focusibility at any number of turns, theoretically speaking, the multi-turn ion optical system is required to satisfy a very strict condition which is called the “perfect focusing condition” under which not only ions are temporally focused at the focusing point but the deviation and angle of the orbit of the ions are the same before and after the flight along the loop orbit. Designing an ion optical system that satisfies this condition is very difficult, and even if it can be designed, it will be awkward with little flexibility in the arrangement and size of the optical elements.
[Patent Document 1] Japanese Unexamined Patent Application Publication No. H11-195398
[Non-Patent Document 1] M. Toyoda and three other authors, “Multi-turn time-of-flight mass spectrometers with electrostatic sectors,” Journal of Mass Spectrometry, 2003, 38, pp. 1125-1142
[Non-Patent Document 2] S. Uchida and five other presenters, “Development of a portable Multi-Turn Time-of-Flight Mass Spectrometer MULTUM S,” Abstract of The 53rd Annual Conference On Mass Spectrometry, 1P-P1-28, 2005, pp. 100-101
[Non-Patent Document 3] M. Ishihara and two other authors, “Perfect space and time focusing ion optics for multiturn time of flight mass spectrometers,” International Journal of Mass Spectrometry, 2000, 197, pp. 179-189